The U.S. military directly attacked Houthi rebels in Yemen for the first time on Wednesday — firing Tomahawk cruise missiles at three rebel-held radar stations on the Red Sea coast. The attack, which was in retaliation for a failed missile attack on a U.S. Navy destroyer on Sunday, risks drawing the U.S. further into the 18-month war.

In March 2015, a coalition of states led by Saudi Arabia began a U.S.-backed bombing campaign against the Houthi forces, which four months earlier had seized Yemen’s capital and deposed the country’s U.S.- and Saudi-backed dictator. Since then, the U.S. has flown refueling missions for Saudi aircraft, supplied targeting intelligence, and resupplied the Saudi effort with tens of billions of dollars of weapons.

While the U.S. has previously conducted direct attacks in Yemen against al Qaeda — which controls vast territory in central and eastern Yemen — it had not directly engaged Houthi forces before.

The escalation began last week when the U.S. dispatched warships to the Bab al-Mandab Strait — which connects the Red Sea to the Gulf of Aden — after the Houthis fired on and nearly sank a ship from the United Arab Emirates. The UAE is a part of the Saudi-led bombing coalition, which has maintained a strict naval blockade of the country since the war began.

When the Houthis fired on the U.S.S. Mason earlier this week, sailors were able to deploy countermeasures and the ship was not damaged.

The Department of Defense issued a statement describing the U.S. attack as a series of “limited self-defense strikes,” but promised to “respond to any further threat” to U.S. ships “as appropriate.”

“The intent of our strikes were to deter future attacks and to reduce the risk to U.S. and other vessels,” White House Deputy Press Secretary Eric Schultz said Thursday. “We are prepared to respond if necessary to any future missile launches.”

The U.S. Navy tweeted a video of the destroyer U.S.S. Nitze launching cruise missiles, captioning it with the hashtag “#Yemen” — commonly used by activists to draw attention to the humanitarian catastrophe.

Schultz said the strike was approved by President Obama on the recommendation of the secretary of defense and the chairman of the Joint Chiefs of Staff. Pentagon officials told NPR that they had “no sense of any civilians being killed,” but it is unclear how they know, and what type of review was undertaken.

Houthi forces denied having launched the missiles at a U.S. destroyer, and promised to respond to kind to the display of military force.

The attack came just after the first sign that the Obama administration might be having second thoughts about the massacres committed by the Saudi coalition with U.S. weapons.

Just four days ago, the Saudi coalition bombed the funeral of a rebel-appointed government minister’s father, killing 125 and wounding 525 in one of the worst massacres of the war. Fragments of what appeared to be U.S.-made bombs were photographed at the scene.

The White House responded by promising to initiate a review of U.S. assistance to Saudi Arabia, and issued its first public threat to stop supporting the coalition. “U.S. security cooperation with Saudi Arabia is not a blank check,” Ned Price, spokesman for the White House National Security Council, said in a statement. Price added that the administration is “prepared to adjust our support so as to better align with U.S. principles, values and interests.”

Top photo: A Sea Hawk helicopter lands on the flight deck of the U.S.S. Nitze on August 15, 2012.

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